This survey of magnetization in sediments from Hole 1006A indicates that a continuous reversal stratigraphy is probably unobtainable because of the weak nature of the magnetic remanence and the related problems of drilling/coring overprints (or remagnetization). A reliable polarity determination may be available in some isolated interval of the cored section (although these should be used with due caution).
The reason for a general increase in NRM intensity with depth in Hole 1006A (Fig. 2) is unknown at this time, but it may be linked to the input of detrital (noncarbonate) sediments. Hole 1006A penetrated a thick pile of upper Miocene and lower Pliocene sediments thought to represent current drift deposits. These intervals may include subtle increases in detrital sediments partially responsible for providing the downcore increase in remanence. Geochemical logging at Hole 1006A has been interpreted to reflect an increase in detrital minerals at two intervals (~350-450 mbsf and ~500-650 mbsf) (Shipboard Scientific Party, 1997c). Spectral gamma-ray results indicate a nearly twofold increase in percent potassium, perhaps related to increased abundance of K-feldspars, micas, glauconite, or clay minerals. If this geochemical log signal is responding to a greater detrital component, it would be expected to also contain grains having a stable magnetic remanence. The influence of detrital grains on the magnetic remanence is consistent with positive susceptibilities measured in these two intervals (Fig. 1). Likewise, the detrital intervals in the upper 125 m of Hole 1006A reveal positive susceptibilities and greater whole-core remanence intensities (Shipboard Scientific Party, 1997c) relative to the background carbonate intervals.
The weak remanence documented here does, however, yield some initial insight into the nature and preservation of carbonate sediment magnetization along the platform-to-basin transect. The middle and lower slope deposits comprising the western margin of Great Bahama Bank are weakly magnetized and often magnetically unstable. At Hole 1006A, ~27% of the samples were judged stable (this is likely a minimum percentage because of the drilling overprint problem), whereas cores (Bahamas Drilling Project cores Clino and Unda; Eberli et al., 1997) from the margin of the platform top had percentages of samples with stable remanence of more than 80% (McNeill, 1997). If this difference in magnetic remanence is real and can be confirmed, factors involving preservation of the ultrafine-grained single-domain magnetite and the detrital component need to be explored, especially in the geochemical and diagenetic realm.